1. Field of the Invention
This invention relates generally to data processing systems and more specifically to apparatus for encoding information and shifting clock and data bits for writing on a magnetic surface such as a disk or diskette.
2. Prior Art
The need for higher density recording of digital information on a magnetic medium has fostered the development of numerous encoding schemes including both phase and frequency encoding. With increased data processing speeds placing greater emphasis on high density magnetic recording, resort has been had to frequency modulated (FM) and modified frequency modulated (MFM) binary information streams. The FM and MFM information streams are generally referred to as single density and double density encoded information, respectively.
FM or single density encoding is disclosed in U.S. Pat. No. 4,034,348 to Rathbun. Further, MFM or double density encoding has been disclosed in a Control Data Corporation project note entitled "MFM Double Density FDD System", and dated Nov. 16, 1976. Also disclosed therein is the occurrence of an illegal missing clock in the address mark field which serves as an address mark.
During the generation of FM and MFM encoded information, severe peak shifts may occur in the clock and data bits as a result of magnetic crowding, mechanical jitter and various electrical effects. The peak shift problem may be further aggravated during read recovery. Where the magnitude of the peak shift may be predicted, the encoded data may be precompensated prior to a recording.
The FM and MFM precompensation systems in general have been comprised of frequency sensitive devices such as delay lines, arrays of monostable multivibrators wherein the data stream is rearranged by moving the data bits into one or more flip-flops at a time, or multiphase clock sources.
The delay lines have exhibited a limited accuracy which is unacceptable in contemporary data processing systems, and are not compatible with integrated circuit implementation. Further, the arrays of monostable multivibrators and multiphase clock sources add unneeded complexity to the encoding system, and thereby increase the likelihood of mechanical jitter and electrical effects which cause peak shifts in the data stream.
U.S. Patent Application Ser. No. 862,258 entitled "Write Precompensation System is directed to a system" for precompensating MFM encoded information without the use of delay lines, flip-flop arrays or multiphase clocks. However this solution requires a relatively large number of integrated circuits. Also the precompensation is accomplished under a fixed set of rules.
It should be understood that the references cited herein are those which the applicants are aware and are presented to acquaint the reader with the level of skill in the art and may not be the closest references to the invention. No presentation is made that any search has been conducted by the applicants.